A light emitting diode (LED) is a semiconductor light emitting element that converts electric energy into ultraviolet light and visible light to emit the light, and has advantages such that it has a long operating life with high reliability, and when it is used as a light source, a time which exchanges lamps can be saved. An LED lamp having an LED chip sealed with a transparent resin is used in a backlight of liquid crystal display device, a signal device, switches, a vehicle lamp, and ant illuminating device such as generic illuminations.
As for a color tone of light emitted from the LED lamp, light in a visible range from blue to red according to an intended use can be obtained by combining an LED chip and phosphors having various emission colors. A white light emitting LED lamp (white LED lamp) has rapidly come into wide use as a backlight of liquid crystal display device, a vehicle lamp and the like. Further, since a fluorescent lamp uses mercury, an applicable range of an illuminating device using a white LED lamp that does not use mercury is expected to greatly expand as a substitute for the fluorescent lamp in future.
As white LED lamps currently in actual or experimental use, there have been known an LED lamp in which a blue emitting LED (emission wavelength: 460 to 480 nm) and a yellow phosphor or a mixed phosphor of yellow and red are combined, and an LED lamp in which an ultraviolet emitting LED (emission wavelength: 300 to 430 nm) and a mixture of respective phosphors of blue, green and red (BGR phosphor) are combined. The former lamp is more widely used at present than the latter lamp because the former is superior in brightness characteristic to the latter.
On the other hand, the latter white lamp using the ultraviolet LED has a characteristic such that it is superior in color reproducibility to the former lamp, since it uses three colors of phosphors. Further, when used as an illuminating device, the former lamp can easily obtain a white color, but it has an inferior color rendering property due to a lack of red component, and meanwhile, the latter lamp is expected to improve the color rendering property based on the selection and improvement of the phosphors. The white lamp using the ultraviolet LED is expected to be a mainstream of white LED lamps in future. In particular, there is a great expectation to apply the white lamp to an illuminating device.
When a phosphor that has been commonly used in a conventional fluorescent lamp (FL) or a cold cathode fluorescent lamp (CCFL) excited by mercury gas is applied to a white LED lamp in which a long-wavelength ultraviolet light having an emission wavelength of 300 to 430 nm is an excitation source, the phosphor sometimes does not emit light since the wavelength of excitation source is too long. In particular, a LaPO4:Ce, Tb phosphor used as a green phosphor does not emit light almost at all when being excited by the long-wavelength ultraviolet light having a wavelength of 300 to 430 nm. Accordingly, it has been studied to use a (Ba, Mg) Al10O17 phosphor or the like as a green phosphor in the white LED lamp using the ultraviolet LED (refer to Patent References 1, 2).
However, the (Ba, Mg) Al10O17 phosphor has an emission wavelength of around 515 nm, so that it can obtain only light on the short wavelength side compared with an emission wavelength (543 nm) of the LaPO4:Ce, Tb phosphor that has been used in the conventional FL or CCFL. When a white LED lamp using such a green phosphor is applied to an illuminating device, there is generated a problem such that the color rendering property cannot be sufficiently enhanced due to a lack of yellow component, as compared with the conventional FL or CCFL. From the above reasons, it is desired to enhance the color rendering property of ultraviolet excitation-type white LED lamp by using a green phosphor containing a lot of yellow components.    Patent Reference 1: JP-A 2000-073052 (KOKAI)    Patent Reference 2: JP-A 2002-171000 (KOKAI)